1. Field of the Invention
The present invention generally relates to methods and devices for detecting an amount of actuator offset in an optical disk device, and more particularly relates to a method and a device for detecting an amount of offset of an actuator which is used for moving a light beam spot to a predetermined position on an optical disk in order to record or reproduce signals for the optical disk.
2. Description of the Related Art
Generally, optical disk devices are equipped with an actuator for moving a light beam spot to a predetermined position on an optical disk in order to record or reproduce signals for the optical disk. The actuator includes a lens actuator (fine actuator), which performs a fine adjustment of a position of the beam spot by means of optical lens. Also, the actuator includes a voice coil motor (VCM, or coarse actuator), which performs a rough adjustment of the position of the beam spot by moving a carriage supporting the lens actuator in a radial direction of the optical disk. Both actuators are movable in the radial direction of the optical disk.
Each of the actuators is powered by an electric current from a driving circuit through wiring cords mounted on a flexible printed circuit sheet (FPC) or the like. The flexible printed circuit sheet with its flexibility urges the actuators to one side in its direction of movement. In other words, the actuators are subjected to an offset force in the moving direction. Also, depending on the positioning of the disk device, the actuators are subjected to a positive or negative offset force owing to gravity.
Accordingly, when the electric current is applied to accelerate or decelerate the actuator, the acceleration or deceleration of the actuator will be different from a desired magnitude due to the offset force. Especially, a seek operation for moving the beam spot to a predetermined track on the optical disk through acceleration or deceleration of the VCM will be affected by the offset force. Since the moving path of the carriage is relatively longer than the moving path of the lens actuator, a moving speed of the carriage is determined according to the length of the moving path. However, the actual moving speed of the carriage will be different from the determined one depending on tension of the FPC and the positioning of the disk drive, so that a stable seek operation will not be obtained.
In order to counter this problem, the offset amount can be estimated in advance such that the electric current applied to the actuator can be adjusted based on the magnitude of the offset force. However, the magnitude of the offset force is slightly different from disk device to disk device. Also, it will be affected by the positioning of the disk device. Thus, the estimate of the offset force is not accurate. Thus, it is desirable that the magnitude of the offset force be measured from the time when the optical disk is inserted into the optical disk device to the time when the optical disk device is ready for operation.
A method of measuring the magnitude of the offset force can be as follows. First, electric current is applied to press the actuator against a stopper provided at one end of its moving path. Next, the amount of the electric current is gradually changed while an output of a position sensor for detecting the position of the actuator is monitored. Then, the amount of the electric current applied to the actuator when the actuator just starts moving is measured. Finally, the magnitude of the offset force can be determined based on the measured electric current.
However, since the electric current needed for pressing the actuator against the stopper cannot be known in advance, an excess amount of electric current must be applied at first. The problem is that applying the excessive amount of the electric current might result in a crash of the actuator depending on its positioning, thus causing damage to the actuator.
Also, the estimation of the offset amount described above requires moving the actuator to the end of its moving path and gradually changing the amount of the electric current until the actuator starts leaving the end position. Thus, the estimation of the offset force takes a long time, which in turn leads to a long delay before the optical disk device is ready for operation.
Accordingly, there is a need for a method and a device for detecting an actuator offset amount which can accurately measure an offset amount in a short period of time.